Aimed at the problem that the viscosity of biodegradable synthetic ester oil changes with the varying environmental parameters under deepening environment,such as temperature and pressure,the deepening environment model was established with environmental pressure and sea water temperature as environmental parameters,and the model of oil viscosity was deduced by using the “modal equation”.By analyzing the error of theoretical results of ISO VG 32 and ISO VG 46,their maximum errors are obtained by 69% and 84%,respectively,which verifies the validity of the model.Based on the above model,a theoretical analysis model of oil viscosity using sea depth as the independent variable was proposed,which reveals threestep dynamic change rules of oil viscosity with the increasing of dive depth,including increasing rapidly,transition adjusting and increasing steadily,respectively.The changing rule of oil viscosity characteristics under deepening environment lays theoretical foundations for selection of underwater hydraulic medium and analysis of the performance of hydraulic equipment in deepening environment.